Color differences jpeg and png

[DearDeparted]

I'm puzzled by the color difference between an exported jpeg (dull) and a png (bright) and hope someone can help me understand what is going on.

I'm maintaining a website and part of it is uploading images created from a (sRGB) psd file from the designer. I edit some text inside that psd and export it for uploading to the website.

The previously uploaded images from this psd by others are all png images (bright).

What I see happening is this:

1. On my monitor I see the colors as they should be and like the images on the website
   
2. Exporting the image in png is the same
   
3. Exporting the image as jpeg quality 100 gives the same result as the png when viewed in a color managed image viewer like Geeqie, but dull in the default Linux Mint image viewer which has no color management
   
4. Uploading the png is okay, same bright colors
   
5. Uploading the jpeg gives dull colors.
   

I would expect the png and jpeg to be the same. All are simple sRGB images. So what am I missing here?

I'm using a Dell monitor and calibration via Spider 4.